1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method. More particularly, the present invention relates to an image forming method and apparatus in which optical sensors detect an amount of light with respect to an endless moving member so as to adjust image skew, image shift, misregistration or other variables.
2. Description of the Related Art
Background image forming apparatuses have employed an optical sensor unit to detect optical characteristics of an endless moving member such as an intermediate transfer belt. The optical sensor unit has a holding member that holds a plurality of optical sensors arranged in a direction perpendicular to a moving direction of the endless moving member. The, plurality of optical sensors detect respective amounts of light reflected therefrom or transmitted therethrough at different positions on a surface of the endless moving member. A background image forming apparatus with such optical sensor unit calculates respective amounts of image skew and image shift on the surface of the endless moving member, based on the results of the detection of the optical characteristics.
One background image forming apparatus with the above-described optical sensor unit is well known to have the above-described technique to detect changes in amounts of reflected light on a surface of a transfer belt that serves as an endless moving member with two optical photosensors, one of which is disposed at one end portion in a widthwise direction of the transfer belt and the other of which is disposed at the other end portion in the widthwise direction thereof. Based on the detection result, the background image forming apparatus determines the respective positions of the leading edges of respective toner images transferred at both ends in the widthwise direction of the transfer belt to obtain respective amounts of image skew and image shift of the toner images.
In another background image forming apparatus having a tandem system in which respective toner images of different colors are formed on a plurality of photoconductive drums and are transferred onto a transfer member in an overlaying manner, reference toner images with respective toners of different colors are transferred onto the surface of a transfer belt. That is, the respective toner images plotted by predetermined pixel patterns are transferred onto the transfer belt to be arranged in a widthwise direction of the transfer belt, or in a direction perpendicular to a moving direction of the transfer belt. A plurality of optical sensors are arranged on the surface of the transfer belt along the widthwise direction of the transfer belt so as to detect amounts of adhered toner per unit area with respect to the respective reference toner images. Based on the amount of reflected light obtained by the plurality of optical sensors, the amounts of respective toners to be supplied to respective developing units of the background image forming apparatus are adjusted.
Since the plurality of optical sensors arranged in the widthwise direction of the transfer belt can detect the amounts of respective toners at different positions with respect to the respective reference toner images at the same time, the image forming apparatus with the plurality of optical sensors can perform the replenishing of toner more quickly than an image forming apparatus with a single optical sensor sequentially detecting the amount of toner to be supplied with respect to respective toner images of different colors of toner.
It is important to arrange a plurality of optical sensors with accuracy, along a direction perpendicular to the moving direction of the surface of an endless moving member.
When the plurality of optical sensors are not properly disposed, the amounts of image shift and image skew cannot be accurately detected. Further, when the plurality of optical sensors are not disposed at identical distances with respect to the endless moving member, the proper amounts of toner adhered on the surface of the endless moving member cannot be detected, which may lead to an incorrect adjustment of toner density. Therefore, it is strongly desired that the plurality of optical sensors are accurately disposed at predetermined positions so as to prevent errors in respective relative positions thereof and respective distances thereof with respect to the surface of the endless moving member.
The cause of the above-described errors, however, may depend on the structural layout of the above-described background image forming apparatuses. In maintenance of an image forming apparatus, for example, when a door mounted on a housing of the image forming apparatus is opened, it is difficult for a technical representative to fully perform a visual check of the optical sensors disposed inside the image forming apparatus. More specifically, the technical representative can see one side of a frame or a board holding the optical sensors but cannot see the other side, or simply cannot see the entire frame or board. Therefore, the person needs to take the entire frame or board of the optical sensors out of the image forming apparatus so that he or she can fully check the respective positions of the optical sensors. When the technical representative reattaches the frame or board of the optical sensors to the image forming apparatus after the check has been finished, however, it is difficult to tell that the optical sensors are precisely mounted as they were before the check. If the frame or board has not been positioned with accuracy, the optical sensors may easily be displaced with respect to the endless moving member, and thereby the image shift and image skew can easily be caused.